The invention relates to a method for cold-running operation, that is during warm-up, of a spark-ignition direct-injection 4-stroke internal combustion engine.
In motor vehicles, in particular in passenger cars, spark-ignition, 4-stroke internal combustion engines, which are also referred to as Otto engines, are widely used. Such internal combustion engines are increasingly being provided with fuel injectors by means of which fuel is injected directly into the cylinders of the internal combustion engine. A fuel/air mixture is formed with fresh air which is sucked into the cylinder, and said fuel/air mixture is ignited at a predefined time by means of a spark plug in order to initiate a combustion process.
The exhaust gases which are produced contain pollutants which will not meet relevant regulations unless they are treated before they are discharged. Under certain operating conditions however, the fuel/air mixture is not completely burnt in the cylinders. The exhaust gas which is produced contains portions of hydrocarbons, carbon monoxide and nitrogen oxides, for the conversion of which into substances which are more compatible with the environment an exhaust gas catalytic converter is connected downstream in the exhaust gas system. The effectiveness of the exhaust gas catalytic converter depends on its operating temperature; it only starts to become effective above a certain threshold temperature, the so-called “light-off temperature”.
After a cold start and during the subsequent cold-running operation, the mixture can only be prepared unsatisfactorily in the cold cylinders or combustion chambers of the internal combustion engine. The exhaust gas catalytic converter which is also still cold or not sufficiently warmed up cannot convert the high emissions of hydrocarbons and carbon monoxide which are produced, or can only convert them to an insufficient degree.
After a cold start occurs and in the subsequent cold-running operation, the objectives are to achieve both quiet running of the engine despite the cold engine components and to heat up the exhaust gas catalytic converter quickly. In conventional operating methods, the engine is for this purpose operated after the cold start with a rich mixture charge in order to ensure a sufficiently reliable combustion for an acceptable level of smooth operating performance. A late ignition time is aimed at, the late timing of which is, however, limited by the combustibility and also the operating noise of the engine. The rich mixture gives rise to high proportions of unburnt fuel in the exhaust gas, which can be used to heat the still cold exhaust gas catalytic converter. By adding secondary air to the exhaust gas, a thermal post-combustion can be obtained, the reaction heat of which accelerates the heating-up of the exhaust gas catalytic converter. It is, however, disadvantageous that it is necessary herefor to use an electrically operated secondary air pump or a secondary air charger as well as a plurality of electrical and pneumatic switching valves. The operation of these components has to be monitored, which is costly. And in any case, good emission values and smooth running of the engine are not achieved.
It is the principal object of the present invention to provide a method for cold-running operation of a spark-ignition 4-stroke internal combustion engine, in which method rapid heating of the exhaust gas catalytic converter is achieved in conjunction with a good level of smooth running and low emission values.